The present invention relates generally to optical disk recording methods and devices for recording desired information onto optical disks, such as CD-R, CD-RW, CD-WO, MD and DVD types, using light power, and more particularly to tracking servo control and wobble signal detection during recording.
Conventionally, in the pitch-forming tracks of write-once optical disks such as CD-R, CD-RW, CD-WO, MD and DVD types, there are formed grooves called “pregrooves”, which have pre-recorded therein absolute time information also known as ATIP (Absolute Time In-Pregoove). Specifically, the ATIP information is embedded in the optical disk by wobbling the track grooves, and necessary recording/reproduction control is performed on the basis of the ATIP information that is obtained by reproducing signals representative of the track wobbles (hereinafter called “wobble signals”).
Tracking error and wobble signals are generally detected by processing outputs from a position-detecting photodetector incorporated in an optical pickup. Specifically, as illustratively shown in FIG. 7, the position-detecting photodetector of the optical pickup comprises a four-quadrant or four-part photodiode 101 which is so named because it has a set of four light receiving surfaces A, B, C and D. Two combinations of the surfaces (A+D) and (B+C) form two composite light receiving surfaces that are segmented in the radial direction of the optical disk along a demarcation line extending along the tracks, and other two combinations of the surfaces (A+B) and (C+D) form other two composite light receiving surfaces that are segmented in the radial direction of the optical disk along another demarcation line extending in a direction transverse to the tracks. Reflected light detection signals, produced by the light receiving surfaces A, B, C and D receiving a light beam reflected from the optical disk, are sampled and held by respective sample-and-hold circuits 102a, 102b, 102c and 102d. The reflected light detection signals, output from the light receiving surfaces A and D and then sampled and held by the sample-and-hold circuits 102a and 102d are added together by an adder 103, while the reflected light detection signals, output from the light receiving surfaces B and C and then sampled and held by the sample-and-hold circuits 102b and 102c, are added together by another adder 104. Then, a subtracter 105 calculates a difference between the sums from the adders 103 and 104, i.e., “(A+D)−(B+C)”. EFM (Eight to Fourteen Modulation) signals modulated on the basis of presence/absence of pits in the optical disk are each detected in the same phase by the light receiving surfaces A, B, C and D. But, the (A+D) and (B+C) signals are detected in opposite phases by the two composite light receiving surfaces that are located opposite to each other in the radial direction of the beam spot on a land of the optical disk, so that they can be used as the tracking error (TE) signal or used for detection of the wobble signal.
Generally, the optical axes of optical pickups employed in this type of optical recording device are factory-adjusted after manufacture of their associated disk drives; however, because the optical axes are not necessarily correctable to an ideal complete condition, the optical axis adjustment is usually judged to be acceptable as long as they fall within a permissible range (manufacturing tolerance) predetermined for the disk drives. Further, in the manufacture verifications, it is desirable that all the optical pickups be judged to be acceptable after having undergone all possible tests based on all possible data. In practice, however, only minimum necessary verifications are carried out on the basis of so-far accumulated performance records and statistical and technical evidences, in order to meet market demands for prompter shipment and lower price. Thus, as the factory adjustment for verifying the manufacturing accuracy of the optical axes, it has been conventional to actually record data onto disk media by means of the manufactured disk drive and then read out the thus-recorded data from the disk media using the same disk drive to thereby verify the reproduced signal quality. The reproduced signal quality is normally determined by measuring jitters in the reproduced signal and then ascertaining whether the measured jitters are within a permissible range.
What is to be noted here is that the measured jitter values are relative values indicative of variations in the length of recorded pits and optical axis deviations can not be completely corrected by any means possible, i.e., would be left unremoved under any circumstances. FIG. 8 is a diagram showing a typical relationship between the light receiving surfaces of the four-quadrant photodetector and a reflection of the light beam spot received by the photodetector, to explain the optical axis deviations or offset from the center of the photodetector. Assuming that the reflection of the light beam spot has a uniform light intensity throughout its circular region, there would occur a tracking error (TE) signal with an intensity proportional to a total area of the received light on the four-quadrant photodetector. If the radius of the received light beam spot is given as “r” and the deviations, in x and y directions, of the center of the beam spot off the center of the four-quadrant photodetector are given as “a” and “b”, respectively, the respective areas SA, SB, SC and SD of the received light beam on the four light receiving surfaces can be expressed as follows:SA=(1/4)πr2−ar+b(r−a)SB=(1/4)πr2+(a+b)r+abSC=(1/4)πr2−br+a(r−b)SD=(1/4)πr2−(a+b)r+ab  Equation 1Therefore, the tracking error (TE) signal can be expressed asTE∝(SA+SD)−(SB+SC)∝4ar  Equation 2
Namely, this means that a tracking offset or error, as represented by “4ar” of Equation 2, is left unremoved. If recording is performed with a large tracking offset, the reproduced signals will present bad jitter measures. Because of this, optimum jitter measures can be said to represent an optimum recording condition. However, in a situation where the recording is performed with a slight stabilized tracking error, the pits may be formed to relatively uniform lengths and shapes and thus there is a sufficient likelihood that the signals reproduced from the optical disk will present optimum jitter measures.
Further, in order to meet the market demands for prompter shipment and lower price, the factory adjustment may be performed to correct the angle of a disk-rotating motor shaft in such a manner that reproduced signals obtained by reproducing, at a normal speed, data recorded on the optical disk at the normal speed can present jitter values falling within the permissible range. In such a case, the optical axis offset can not necessarily be removed completely (i.e., to a zero value) although the optical disk drive can shipped after having been corrected to a best jitter mode. Although such a verifying approach has not presented significant problems in optical disk recording at not-so-high speeds such as a four-times recording speed, it has been found that the residual optical axis deviations are not negligible either.
Japanese Patent Application Laid-open No. HEI-8-235617 discloses a method, in accordance with which a light-transmitting flat plate is pivotally positioned in the optical path and the optical axis offset is corrected by adjusting the angle of the flat plate. However, the light-transmitting flat plate can not be incorporated in the movable optical pickup and therefore has to be provided separately from the optical pickup, so that the optical path becomes considerably long. Thus, the disclosed method would require higher adjustment accuracy, such as accurate collimation of the laser light beam and accurate optical axis angle, which involves an increased number of component parts and hence an increase in costs. In addition, readjustment would also become necessary due to deviations over time.